Asphalt is presently commonly used for paving streets and highways, tennis courts, driveways, and other outdoor and indoor surfaces. Asphalt pavement has proven to be generally inexpensive to manufacture and install and yet substantially durable for withstanding heavy loads, such as heavy trucks and for withstanding severe outdoor elements. Asphalt plants can be stationary or mobile as may economically be needed for manufacturing and delivering hot asphalt a reasonable distance whereat it is installed with paving equipment.
The production of asphalt first requires preheating of the aggregate to a desired temperature, typically 200-800 degrees Fahrenheit. During this heating process, substantially all moisture is removed from the aggregate. Thereafter, the hot aggregate is mixed with tar for creating the asphalt. Typically, a separate mixing drum is provided for mixing the aggregate and tar although some asphalt plants incorporate a single drum having a heating/drying section and a mixing section.
During repaving and other construction operations, reclaimed asphalt pavement or products (RAP) is created and requires the disposal or reuse thereof. Due to environmental hazards and high costs of disposal, it is undesirable to merely landfill the RAP. It is obviously more desirable to reuse such reclaimed asphalt products thereby eliminating disposal costs while increasing productivity. Similar to the process using virgin aggregate, however, the RAP must first be heated for properly mixing with additional tar and aggregate and thereby manufacturing a correctly mixed useable asphalt.
In the past, RAP has typically been introduced directly into the mixing drum or mixing area along with the hot aggregate and tar. Unfortunately, such a method requires that the aggregate be heated to a much higher temperature so as to compensate for the cold RAP. Additionally, such method requires substantially more mixing so that the RAP can first be heated and thereafter be properly mixed.
Attempts have been made to introduce RAP directly into the aggregate drying drum. However, this is problematic in that, quite often, the RAP begins to liquify thereby dirtying the aggregate drying drum and reducing its efficiency. Additionally, the heating of RAP creates hydrocarbon, carbon monoxide and nitrogen oxide emissions which are unacceptable and must be treated prior to releasing to the atmosphere.
In yet another attempt to pre-heat RAP, a collar has been incorporated circumscribing the aggregate drying/heating drum and a plurality of holes placed in the drying drum for allowing the heated aggregate to fall out of the drying drum and into the outer collar. RAP is introduced into the collar and, therefore, the RAP and aggregate are caused to mix within the annular area between the collar and the drying drum. In these apparatus, although the RAP is preheated and mixed with aggregate prior to being introduced into the mixing drum, similar to merely introducing the RAP directly into the drying drum, hydrocarbons, carbon monoxide and nitrogen oxide emissions are again emitted into the drying drum and require treatment for meeting EPA standards. Additionally, the portion of the drying drum after the aggregate falls out of the drying drum and into the collar is exposed to direct heat from the burner. This unfortunately overheats the drying drum causing deterioration and warpage thereof.
Accordingly, a need exists for an apparatus and method whereby RAP can be preheated efficiently and economically in asphalt plants without damaging the asphalt plant components and without creating undesirable emissions.